The present invention relates to a solid state imaging device, and more particularly to a solid state imaging device usable under high-temperature atmosphere or radioactive ray.
A charge coupled device developed in 1970 by Bell Laboratories has been promoting the development of solid state imaging devices. In addition, use of a silicon substrate in solid state imaging devices has enabled to improve rapidly productivity. Now, the solid state imaging device has started to be used in a wide range of applications from apparatus for consumer use such as a video apparatus, an office computer, a facsimile to machines for industrial use as machine's eye such as measuring instruments and manufacturing machines.
A solid state imaging device of the conventional type comprises an imaging part composed of a photodiode having pn junction on a silicon semiconductor crystal substrate, an accumulation part for accumulating electric charge as an image information, and a charge transfer part transferring electric charge for information processing.
Recently, there has been a tendency to use a solid state imaging device as a monitor or the like in a severe environment as in a blast furnace in which ambient temperature is high, in a location which receives intensive radioactive ray in a nuclear power plant, or under such a sever environment that blizzard is blowing in the Arctic or temperature change is remarkable in a desert. Therefore, the solid state imaging device needs to withstand a high-temperature or intensive radioactive ray.
However, semiconductor devices using silicon crystal are said to have an upper temperature limit of about 140.degree. C. for practical use. (Refer to "Semiconductor Engineering" by Shirafuji, publication of Kyoritsu Shuppan Co., Ltd, pp. 30-35; called reference A, hereinafter). This is because the energy gap .epsilon..sub.g of silicon is as small as 1.1 eV. If this value can be increased, the semiconductor device would be able to operate normally even at a higher temperature. Accordingly, there is a problem that the solid state imaging device using silicon crystal cannot operate normally at a temperature of 140.degree. C. or more.
Furthermore, application of radioactive ray to the semiconductor device causes a so-called "single event" phenomenon to degrade device characteristics. In the single event phenomenon, radioactive ray causes to generate pairs of electron positive hole which are then collected to an electrode in a short time to locally increase in potential, causing malfunction. The resistance against the single event phenomenon increases as the density of generated charge decreases. To decrease density of generated charge, semiconductor crystal needs to have a smaller atomic density, smaller mean atomic number and larger band gap. (Refer to "Basic Research for Development of Radiation-Resistant Electronic Device" by Nashiyama, transactions of The Institute of Electorical Engineer's of Japan 1991, pages S.4-11 to S.4-14 called reference B, hereinafter).
In view of the above-mentioned circumstances, an object of the present invention is to provide a method for producing a solid state imaging device using a semiconductor crystal having an excellent thermal stability and radioactive ray resistance property, which method is capable of realizing mass production of the device.
Another object of the present invention is to provide a solid state imaging device excellent in heat resistance and radioactive ray resistance.